Target of Opportunity observations are described in section 3.1.3 of the second Announcement of Opportunity:
A Target of Opportunity (ToO) is an astrophysical phenomenon which cannot be predicted and for which observations cannot be proposed in the normal manner. Target of Opportunity proposals may be submitted at any time, in the same format as normal proposals, to the VSOP Project Scientist. Before submitting a ToO proposal, proposers should use the simulation software packages to ensure the source is in a region of sky which HALCA can observe. Proposals should also indicate the timescale for which ToO observations are requested. A minimum time period of four days should be assumed from receipt of a ToO proposal to the earliest possible observation being made. ToO proposals requiring a rapid (i.e. less than 2 week) change of HALCA and ground radio telescope schedules will be permitted only in exceptional circumstances. ToO observations will require the approval of the VSOP Project Scientist and his VISC co-chair. Given the time-critical nature of these observations, Target of Opportunity proposals may be submitted in their entirety by e-mail and/or fax...
Refer to the full Announcement of Opportunity for more details.